If you're aiming for $750,000 by age 67, here's exactly how much you need to set aside per month. The chart, provided by NerdWallet, assumes a 6 percent average annual investment return. The green bars represent how much you need to save and the blue bars represent your investment return. Hover over the bars to see the exact numbers.

Here's the full breakdown of how much you would have to save per month to have $750,000 by age 67 if you started at age:

20: $239 per month25: $330 per month30: $460 per month35: $648 per month40: $930 per month45: $1,373 per month50: $2,123 per month

As the chart shows, the amount you have to save depends a lot on how early you start. If you begin setting aside money at age 20, you have to save $239 per month to reach $750,000. If you start at age 30, though, that number nearly doubles: You need to save $460 per month.

"The difference between starting at age 20 and starting at age 50 is striking," Arielle O'Shea, NerdWallet's investing and retirement specialist, tells CNBC Make It. "Someone who consistently saves from age 20 is able to build a $750,000 balance by saving nearly 90 percent less per month than someone who gets a later start at age 50."

That's thanks to compound interest. Compounding makes a sum grow at a faster rate than simple interest, because in addition to earning returns on the money you invest, you also earn returns on those returns over time.